Variable band width piezoelectric filter



Sept. 26, 1939. J. B. ATWOOD 2,173,894

VARIABLE BAND WIDTH PIEZOELECTRIC FILTER Filed Feb. 1, 1938 I5 LSECT/O/VA SECTION B *G *5 v 1 1 .2 VAR/ABLE F/XED LOSS 9 O r :2 Q [ARMSY 1 g I t FREQ m I g I 55/2/55 ARMS I K I l k Q 1 4 .3

+ FIXED VAR/ABLE v) LOSS LA F g I 1 E5 ARMS 9 k l g INVENTOR i J. 5.ATWOOD ATTORN EY Patented Sept. 26, 1939 UNITED STATES PATENT OFFICEFILTER John Boyd Atwood, Riverhead, N. Y., assignor to Radio Corporationof America, a corporation of Delaware Application February 1, 1938,Serial No. 188,034

5 Claims.

This invention relates to piezo electric crystal filter systems and moreparticularly to a filter employing quartz crystal resonators in whichvariability of the band width is obtained.

It is an object of my invention to provide a filter system of the typementioned in which fixed peaks of attenuation at the edges of a desiredpass band are obtained.

It is another object of my invention to provide a filter system havingtwo attenuation regions, one extending from zero frequency to the lowfrequency edge of the pass band, and the other extending from the highfrequency edge of the band to infinite frequency.

Another object of my invention is to provide a piezo electric filtersystem having two variable attenuation bands which may be caused to varyin extent thruout the region of the pass band and which produce avariable band width feature.

Still another object of my invention is to provide a piezo electricfilter system having a variable pass band width which exceeds the limitof 13.5% heretofore considered a maximum.

Other objects and advantages of my invention will be made apparent inthe description to follow. This description is accompanied by a drawing,in which Figure 1 shows a schematic wiring diagram of a filter systemhaving two sections the elements of which are suitably designed andco-ordinated for carrying out my invention,

Fig. 2 shows diagrammatically a system of reactance curves whichcorrelate the reactance or loss in one section of a lattice network withfrequency; and

Fig. 3 shows a system of reactance curves which correlate reactance withfrequency in the second section of the lattice network of my invention.

When dealing with piezo electric crystal filters of the prior art it hasbeen noted that there is a maximum limit to the fixed band width ofapproximately 13.5% of the mid-frequency. Once the filter has beendesigned for a fixed band width this width cannot be varied. The reasonfor this is that any attempt to vary the width of the band results inspurious pass bands in the desired attenuation region or undesired lossbands in the desired pass band, or both. In carrying out my invention Ihave found it possible to overcome these difficulties by a suitablearrangement of the circuit elements themselves.

Referring to Fig. 1, it will be seen that the filter consists of a twosection lattice arrangement connected by an impedance matchingtransformer I which merely eliminates such losses as would be caused bythe impedance mismatch at the juncture of the two sections. Each sectioncomprises two series-arms and two crossarms; hereinafter referred to aslattice-arms.

The series arms of section A are made up of the matched quartz crystalsl4 and I5 shunted by the fixed condensers 6 and l. The lattice arms aremade up of two identical resonant circuits,

one being 2 and Ill and the other being 3 and H. 3

The series arms of section B are made up of the matched quartz crystals16 and I! shunted by the fixed condensers 8 and 9. The lattice arms aremade up of two identical anti-resonant circuits, one being 4 and I2 andthe other being 5 I and 13.

In Figure 2 are shown the reactance curves and the loss Vs. frequencycharacteristics of section A. It will be seen that the reactance curvefor the series arms of the section is that of a resonant circuit shuntedby a capacity. This is produced by the quartz crystals l4 and l 5. Thecondensers 6 and l shunting the crystals are for the purpose of makingthe curve extend further to the left below the axis so that thereactance curve of the lattice arms will intersect it at only a shortdistance from the point where the lattice arms go through resonance. Anattenuation peak is produced where the two curves intersect, since thelattice type of structure is essentially a Wheatstone bridge and at thecrossover point the bridge is balanced. A fixed attenuation band isproduced between the resonant and anti-resonant points of the crystaland this is so located that it is at the high frequency limit of theband for which the filter is designed.

A loss band. is produced from zero frequency to the resonant point ofthe lattice arms. The location of the cut-off point of this band isvaried by changing the capacities of the variable condensers 2 and 3.Since the portion of the reactance curve of the series arms below theaxis is nearly horizontal over the range where the lattice arm curvecrosses it, the frequency separation between the cut-off point and theattenuation peak will remain nearly constant as the band width is variedon the low frequency side.

In Fig. 3 are the reactance curves and the loss vs. frequencycharacteristics of section B. It will be seen that the resultantattenuation curves are the reverse of those for section A.

The reactance curve for the series arms of the section is that of aresonant circuit shunted by a capacity. This is produced by the quartzQrystals l6 and H. The condensers 8 and 9 shunting the crystals are forthe purpose of making the curve nearly horizontal on the high frequencyside of the anti-resonant frequency. This is done so that the reactancecurve of the lattice arms will cross it at only a short distance fromthe anti-resonant frequency of the lattice arms. An attenuation peak isproduced where the two curves cross. A fixed attenuation band isproduced between the resonant and anti-resonant points of the crystaland this is so located that it is at the low frequency limit of the bandfor which the filter is designed.

A loss band is produced from the anti-resonant frequency of the latticearms to infinite frequency. The location of the cut-01f point of thisband is varied by changing the capacities of the Variable condensers 4and 5. Since the portion of the reactance curve of the series arms belowthe axis is nearly horizontal over the range where the lattice arm curvecrosses it, the frequency separation between the cut-off point and theattenuation peak will remain nearly constant as the band width is variedon the high frequency side.

In operation, the four variable condensers 2, 3, 4, and 5 may, ifdesired, be ganged together and controlled from a single dial to producethe variation in band width which is sought.

It is readily Within the abilities of any one skilled in the art tofollow the teachings hereinabove given by way of modifying the design ofthe filter sections, particularly so as to provide impedance matchingtransformers at each end of the filter in addition to the ones betweenthe filter sections. In each case loss vs. frequency characteristics ofthese transformers may be so designed as to improve the losscharacteristics of the filter in the regions outside the pass band.

Other modifications of my invention will doubtless suggest themselves tothose skilled in the art. The invention itself is, therefore, to begiven the broadest interpretation that is permitted by the scope of theclaims to follow.

I claim:

1. An electric wave filter of variable band pass characteristics havingtwo intercoupled impedance network sections including series-arms andlattice-arms, a piezo electric device in each of the series arms of saidsections, means including a capacitor in shunt with each of said piezoelectric devices for minimizing the frequency separation between thecut-off point and the variable attenuation peak of each section, and forkeeping this frequency separation substantially constant as the locationof the cut-off point is varied, each lattice arm of the first sectionincluding an inductance in series with a variable capacitor, eachlattice arm of the second section including an inductance in parallelwith a variable capacitor, means associated with the first section forvarying the cut-off point on the low-frequency side of the pass band,and means associated with the second section for varying the cut-offpoint on the high-frequency side of the pass band.

2. A filter in accordance with claim 1 and further characterized in thatthe lattice arms of the first section are substantially resonant at thelow frequency edge of the pass band, while the lattice arms of thesecond section are substantially anti-resonant at the high frequencyedge of the pass band.

3. A variable band electric wave filter comprising two intercoupledsections, each section having two series branches and two latticebranches, each series branch of the two sections havin therein a piezoelectric device and means including a capacitor in shunt therewith forminimizing.

the frequency separation between the cut-off point and the variableattenuation peak and for keeping this frequency separation substantiallyconstant as the location of the cut-off point is varied, said seriesbranches having substantially similar resonant characteristics, meansincluding an inductance in series with a variable capacitor in each ofthe lattice branches of the first section for controlling the cut-offpoint on the low frequency side of the band to be passed, and meansincluding an inductance in shunt with a 7 variable capacitor andconstituting an antiresonant circuit for controlling the cut-off pointon the high frequency side of said band.

4. A filter in accordance with claim 3 and having a variability of itsband width in excess of H 13.5% of the mid-band frequency.

5. A variable band pass filter comprising two intercoupled sections,each section having two lattice branches and two series branches, thelattice branches of the first section having means including aninductance in series with a variable capacitor for independentlydetermining the position in the frequency spectrum of the cut-off pointon the low frequency side of the pass band, the lattice branches of thesecond section having means including an inductance in shunt with avariable capacitor for independently determining the position in thefrequency spectrum of the cut-off point on the high frequency side ofthe pass band, each of the series branches of the two sections havingtherein a piezo electric device and means including a capacitor in shunttherewith for providing a minimum frequency separation between thecut-off point and the attenuation peak on the one side of the pass band,and on the other side respectively, and means for independently varyingthe location of said cut-01f points, thereby to provide a filter ofvariable width and of variable mid-frequency response.

JOHN BOYD ATWOOD.

